SUMMARY/ABSTRACT Anthropoid primates (monkeys, apes, and humans) are distinguished from their phylogenetically nearest relatives (lemurs, tree shrews, and rodents) by an elaboration of the cerebral cortex including changes in cellular composition of the cortical circuit. However, we know very little about the function of different cell types in the anthropoid primate cortex partly because of slow adoption of the tools that have been so successful in rodents (cell type-specific optogenetics and imaging). The present proposal utilizes the common marmoset, a small anthropoid primate with a flat cortex, that allows direct adoption of cellular imaging techniques from rodents. We will develop (1) wide-field optical imaging for collecting 2D maps across large swaths (5-10 mm) of the marmoset visual cortex and (2) fast, volumetric two-photon imaging across layers within a cortical column (400 microns^3 volume acquisition). We will collaborate closely with optical engineers who developed wide- field and volumetric two-photon imaging techniques in mice. As a first application of these methods in the marmoset, we measure the functional properties of marmoset object-selective cortex, collecting wide-field maps of object selectivity followed by two-photon imaging of the information communicated between cortical areas by labeled feedforward and feedback projection cells. This project thus builds an imaging platform for launching a long-term research program to understand the role of different cell types in the highly evolved visual cortex of anthropoid primates.